1. Field of the Invention
The present invention generally relates to the art of fabricating objects using machine tools, and more specifically to a Computer Aided Design (CAD) system for unambiguously constructing Feature Control Frames (FCF) and to Computer Aided Tolerance Analysis (CATA), Computer Aided Manufacturing (CAM) and Computer Aided Inspection (CAI) systems for automatically constructing Datum Reference Frames (DRF) for machine parts.
2. Description of the Related Art
A machine part or object made of metal or other material is conventionally formed or machined using a motorized tool such as a press or a milling machine by immobilizing the part in a holding fixture, and engaging appropriate surfaces of the part with forming or cutting tools to move or remove material and thereby form the part into the required shape.
The precision with which a part must be manufactured can be extremely high, with tolerances often expressed in microinches. With such precision comes the need to accurately determine the location of the cutting tool relative the other features on the part. For example, if a hole must be drilled at a certain distance from an edge of a part, means must be provided to establish a frame of reference in which to measure this distance and accurately position the cutting tool.
This need is fulfilled by a Datum Reference Frame (DRF), which is a Cartesian coordinate system relative to which the locations and attitudes of machine part features are defined. Whereas one or more DRFs may be defined in each part, a DRF is not a physical entity, but rather an imaginary construct to which physical features on a part are geometrically related.
A system of standards has been established for dimensioning and tolerancing using DRFs. These standards are presented in a publication entitled "Dimensioning and Tolerancing", ASME Y14.5M-1994, American Society of Mechanical Engineers 1995. Further standards are set forth in a publication entitled "Mathematical Definition of Dimensioning and Tolerancing Principles", ASME Y14.5.1M-1994, American Society of Mechanical Engineers 1995. These publications are incorporated herein by reference in their entirety.
A DRF is defined by a small number, typically three, of specially selected features on a part called Datum Features (DF), which, if engaged by a holding fixture, render the part immobilized. The immobilizing components of a holding fixture or a functional gage can be seen as the inverses of the datum features, and are referred to as Datum Feature Simulators (DFS). The origin, axes, and planes of the DRF constructed with the help of said Datum Features are referred to as Datums.
For example, a planar surface of a part can be used as a primary datum feature to eliminate pitch, yaw, and one degree of translational freedom, with other datum features being used to eliminate the three remaining degrees of freedom which are roll and two additional degrees of translational freedom. The precise geometrical orientations and locations of the remaining features of the part are then controlled relative to the DRF so constructed.
Prior to machining or inspecting a part, a holding fixture or functional gage is produced. The part is clamped in said fixture such that its datum features mate with the datum feature simulators of the fixture, whereby the DRF of the part is brought into alignment with the DRF of the fixture and therefore with the coordinate system of the machine or measuring tool. This enables the features of the part to be reliably machined and inspected using the dimensions specified in the engineering blueprint or formal drawing.
The concept of a DRF can be better understood through the presentation of an illustrative example, which takes a simple part from the concept stage, through all the ensuing drawing, manufacturing and inspection stages.
At the outset, it is useful to make a simple perspective sketch of the part, which is shown in FIG. 1 and designated by the reference numeral 10. Further illustrated are an origin 0 and the X, Y and Z axes of a DRF which will serve to control the location of the part's features. If the part 10 were handled at this point, it would be discovered that it has six Degrees of Freedom (DOF); it can pitch, yaw and roll (three degrees of rotational freedom), and translate in the X, Y and Z directions (three degrees of translational freedom). Since the coordinate system is attached to the part, it goes wherever the part goes, making it clear that coordinate systems also have six degrees of freedom.
The six degrees of freedom are illustrated in FIG. 2. In the particular case shown, pitch is about the Y axis, yaw about the X axis and roll about the Z axis. Translation is indicated by a coordinate system X',Y',Z' which is translated from the coordinate system X,Y,Z by offsets .DELTA.X,.DELTA.Y,.DELTA.Z.
In order to be manufactured, the conceptual part must be defined in a formal drawing or Computer Aided Design (CAD) data base. In addition to creating its general outlines and dimensions, it is important to select certain features to determine the coordinate system responsible for locating and orienting the other features.
The most reliable of such "datum" features are probably (1) the bottom of the part 10 which is designated by the reference character A, which can eliminate pitch, yaw, and one degree of translational freedom in the Z direction as indicated above; (2) a long edge B of the part 10 which can eliminate roll and one more degree of translational freedom in Y; and (3) a short edge C of the part 10 which can eliminate the last degree of translational freedom in X.
These edges are selected to constitute datum features A, B and C respectively, and control the remaining features using ASME Y14.5M tolerancing tools. A formal drawing of the part 10 is shown in FIG. 3, in which Feature Control Frames (FCF) for position and surface profile incorporate all the information required to construct the intended coordinate systems.
As viewed in FIG. 3, the part 10 includes a hole 12 that is to be drilled 2.750 inches from the bottom edge B, and 3.000 inches from the left edge C. The formal drawing includes an FCF 14 for the hole 12, which specifies that the hole 12 is to have a diameter of 1.000.+-.0.020 inches, and that the center of the hole 12 must lie within a cylindrical tolerance zone of diameter 0.015 inches at Maximum Material Condition (MMC) having its axis at the basic location (3.000,2.750). The concept of MMC will be described in detail below.
The FCF 14 further includes datum feature references 14a, in this case to the planar surfaces A, B and C. This specifies that a DRF for the part 10 is to be constructed using the datum features A, B and C in the order or sequence listed in the FCF. The formal drawing, including the FCF 14 with datum feature references 14a, is prepared by the engineer who designs the part 10, and must be adhered to exactly during all stages of the manufacture, inspection, etc. of the part 10.
Assuming that the part 10 is to be manufactured from a rough forged billet, it is secured in a milling machine vise (not shown), and the top surface is cleaned up using a rotary cutting tool 16 as illustrated in FIG. 4. This results in datum feature A, which coincides with the machine's X-Y base plane as soon as the Z axis of the machine's digital readout is reset to zero.
In the same set-up, the front surface is milled perpendicular to A and parallel to the machine's X axis. This results in datum feature B, which coincides with the machine's X-Z base plane once the Y axis of the machine's digital readout is reset to zero, after correcting for the tool radius. Repeating the process for the right hand surface, the datum feature C is produced. FIG. 4 illustrates the results of the process as described thus far.
Finishing the part at this point would consist of milling the remaining two sides, boring the hole 12, flipping the part 10 over, and cleaning up the top. However, if the machine were needed for other work before the job could be finished, and the unfinished part 10 would have to be removed from the machine, the relationship between the part 10's DRF and that of the machine would be lost.
This, however, is where datum features A, B and C come in. Since they define the part 10's reference frame, they can serve to reunite it with that of the machine.
This is accomplished by flipping the part 10 over and putting datum feature A down on the bottom of the vise to eliminate the part's pitch and yaw, after which the opposed surface may be milled to size in accordance with its Dimension origin tolerance of 1.000.+-.0.005.
Next, datum feature A is clamped against the back of the vise with datum feature B face down on the bottom of the vise. By so doing, datum feature A is used to eliminate pitch and yaw in a new way, and datum feature B, for the first time, to eliminate roll, creating the coordinate system in which the rear surface of the part can be milled to size in accordance with its surface profile tolerance.
Next, in preparation for the final operations, pitch and yaw are again eliminated by placing datum feature A back on the bottom of the vise, and roll by clamping datum feature B up against the back of the vise. With the help of an edgefinder to null the Y axis of the milling machine on the back of the vise and the X axis on the highest point of datum feature C, the remaining end of the part 10 can be milled to size in accordance with its surface profile tolerance, and the hole 16 bored in accordance with its position tolerance.
This example illustrates how datum features A, B and C, can be used to bring the DRF of the part 10 into coincidence with that of the machine for each operation, allowing its manufacture to be completed in strict observance of the requirements of the drawing.
When more than one datum feature is referenced in an FCF, they work in concert to construct a DRF, and it is important to recognize the significance of their sequence. In the FCF for position in the example of FIG. 4, the sequence is ABC. This requires that A be used to eliminate pitch and yaw as well as translation in Z, that B be used to eliminate roll and translation in Y, and C to eliminate translation in X. This requirement is called the rule of precedence, and leads to designation of the datum features as "primary", "secondary" and "tertiary".
Since elimination of pitch and yaw is of the greatest importance, the datum feature chosen for this must be large compared to the locations and extent of the features referred to it. Since the elimination of roll is of almost equal importance, the datum feature chosen for this must also be large compared to the locations and extent of the features referred to it. In the case of datum features used only to eliminate degrees of translational freedom, the smaller the better, to reduce uncertainty.
Even if these requirements for datum feature selection appear not to be observed in the drawing, the stated order of the datum features must nevertheless be respected by all who deal with the part, until changed by the design engineer. FCFs may thus not be "interpreted", but must be "read" in the same way by those involved in the manufacturing process as by those involved in the metrology process. Only this can ensure one-to-one correspondence between the DRFs in each operation, without which the possibility of process control feedback collapses.
This concept is illustrated in FIGS. 5a to 5c. These drawings, as well as other figures that will be referenced below, are greatly simplified in that only the dimensions and tolerances which are necessary for understanding the concepts being described are included. Other dimensions and tolerances, although necessary for providing a complete engineering specification for the corresponding part, are explicitly omitted for clarity of description.
FIG. 5a is a formal drawing for a part 20, which has a bottom surface which constitutes a datum feature A, a lower edge which constitutes a datum feature B and a left hand edge which constitutes a datum feature C.
More specifically, in accordance with well defined procedures described in the referenced standards, the datum feature A is intended to eliminate pitch, yaw, and translation in the Z direction, the datum feature B is intended to eliminate roll and translation in the Y direction, and the datum feature C is intended to eliminate translation in the X direction of said part.
The drawing for the part 20 furthermore specifies a hole 22 which is to be drilled at a basic dimension of 3.000 inches from the datum feature C (equivalently 3.000 inches from the Y axis of the DRF), and 2.750 inches from the datum feature B (equivalently 2.750 inches from the X axis of the DRF).
As illustrated in FIG. 5b, it is assumed that the machining blank in which the hole 22 is to be drilled has become parallelogram-like. Assuming this error is within the tolerances governing the exterior of the part 20, and that the part 20 is placed in a requisite drill fixture 24 using datum feature B to eliminate roll in accordance with the procedure cited above, the hole 22 will be drilled at precisely the correct location specified by the drawing as further illustrated in FIG. 5b.
However, if during the inspection process the formal drawing is misread, and the datum feature C is used to eliminate roll, then, when the part 20 is placed in a requisite inspection fixture 26, as illustrated in FIG. 5c, the location of the hole 22 will appear to be displaced from its basic location. The effect of this error may be the decision to scrap a part which fully meets the requirements of the formal drawing. This demonstrates the power of the FCF in this particular case to unambiguously define the desired DRF, as well as the potential for human oversight to corrupt the system.
An important feature of the present invention relates to an ambiguity in the Y14.5.M and the Y14.5.1M standards and their international counterparts as to whether the orientation or the location of secondary and tertiary datum features should be used to eliminate roll relative to a previously fixed primary axis of a DRF. The current standards provide no means to differentiate between the two alternatives, thus leaving the matter open to interpretation and manufacturing companies exposed to the costly consequences thereof.
In a manner similar to FIGS. 5a to 5c, FIGS. 6a to 6d illustrate a case in which the uncontrolled freedom to choose between orientation and location produces the uncertainty referred to above.
FIG. 6a is a greatly simplified formal drawing of a part 30 which is referred to by practitioners in the art as the "hockey puck". The part 30 has a flat surface which is designated as a datum feature A, and a central hole 32 which is designated as a datum feature B. As indicated by the FCF for the four circumferentially spaced 0.500 inch diameter holes 36, their position is to be controlled relative to a DRF constructed using datum features in the order A, B, and C, wherein A eliminates pitch and yaw and translational freedom in the Z direction, and B eliminates translational freedom in the X and Y directions.
The one remaining degree of freedom is rotation about the fixed primary axis (Z axis) passing through the center of the hole 32. This degree of freedom is intended to be eliminated by the slot 34 which is designated as datum feature C. The slot 34 is to have a width of 0.500, and a nominal midplane which contains the axis of the hole 32. The slab-like tolerance zone within which the actual midplane must lie has a width of 0.020 inches.
Whereas either the orientation or the location of the slot 34 could serve to eliminate roll about the axis through the hole 32, the FCF fails to define a preferred alternative, and thus leads to the confusion described below.
FIG. 6b illustrates the part 30 after the hole 32 and slot 34 have been formed. As shown, the slot 34 was formed imprecisely, more specifically offset 0.010 inch above its basic location, but just within the tolerance specified in the formal drawing.
FIG. 6c illustrates the alternative in which the orientation of the slot 34 is used to eliminate rotation about the axis of the hole 32 for the purpose of drilling the holes 36. All degrees of freedom except roll are eliminated by placing the part 30 on a planar Datum Feature Simulator (DFS) (not shown) which engages the surface A, and over an expanding cylindrical DFS 38 extending upwardly therefrom which engages the hole 32.
FIG. 5c further illustrates how roll is eliminated by a DFS 40 in the form of an expanding slab which engages the slot 34. In order for the orientation of the slot 34 to be used to eliminate roll, the DFS 40 must be free to translate perpendicular to its midplane while expanding to fill all available space in the slot 34.
Since the DFS 40 is free to translate, upon coming to rest its upper and lower surfaces end up parallel to the upper and lower surfaces of the slot 34. Thus, the orientation of the slot 34 is used to eliminate roll.
In FIG. 6d, it will be assumed that the four holes 36 have been correctly drilled using the set-up shown in FIG. 6c, and that the finished part 30 is placed in an inspection fixture comprising a planar DFS (not shown) and an expanding, cylindrical DFS 38' which are identical to those described with reference to FIG. 6c.
In FIG. 6d, it will assumed that the formal drawing has been interpreted as indicating that the location, rather than the orientation, of the slot 34 is to be used to eliminate roll. As a result, the DFS for the slot 34 will be an expanding slab whose midplane is fixed at the basic location of the slot 34, and thus contains the axis of the hole 32.
Since the actual slot 34 is offset upwardly from its basic location, the part 30 is caused to rotate clockwise as the now fixed DFS 40' expands in the slot 34 as shown in FIG. 6d.
Using orientation to eliminate roll in the manufacturing process, as in FIG. 6c, and location to eliminate roll in the inspection process, as in FIG. 6d, causes the four holes 36 to appear to be offset from their desired locations. Since there is no means of deciding whether the orientation or the location of the slot 34 is to be responsible for eliminating DFS roll, it is impossible to say whether the part was manufactured correctly and inspected incorrectly, or vice versa.
This ambiguity has heretofore prevented practitioners in the art from providing a formal drawing including feature control frames which unambiguously define the preferred function of datum features. The same omission has further prevented the implementation of a CAD or other computer system which can automatically construct a DRF.